DEVELOPMENT AND EVALUATION OF CONTINUOUSLY WEIGHING MINILYSIMETERS

Micrometeorological methods for measuring evapotranspiration are based on the assumption that the water vapor flux is constant with respect to height within the first few meters above the ground, and consequently are not applicable where an area is not extensively homogeneous. An increasing emphasis on energy and water flux studies in heterogeneous terrain (e.g. urban and hilly environments) has created a need to develop alternative approaches to quantify the latent heat flux. Lysimetry is an established alternative that permits the direct measurement of evapotranspiration from the ground surface and thus can provide accurate point estimates of the latent heat flux in heterogeneous environments. To date, the continuously weighing lysimeters that have been developed and extensively tested are large (surface area generally greater than 2m2), stationary and expensive to construct and maintain. These instruments are also inappropriate for most geographic studies of energy and water budgets in heterogeneous environments because knowledge of the spatial variability of evapotranspiration and quantification of the horizontal transport of moisture and heat require multiple, simultaneous flux determinations. This research tests two continuously weighing, portable mini-lysimeters (< 0.2 m2) designed for practical application in heterogeneous environments. The performance of the lysimeters are compared with evaporative flux measurements obtained using eddy correlation instrumentation from an extensive homogeneous surface. The soil moisture status of the monolith and the stomatal resistance of the plants within the lysimeter are compared with simultaneous measurements from the surrounding grass field. The results show that the evaporative flux, soil moisture and stomatal resistance measurements from both lysimeters compare favorably with measurements that are representative of their surroundings.